The most suspicious one is the one that acts all hesitant while pointing out more stuff that will be even more incriminating. And when confronting the person, they start to cry and act like they're a victim of this or some righteous person that is driven to tears over such a horrible thing.
So I very much think that this girl is the one that is the one that took that photo in the first place. And likely is the one that asked the other girl to talk to Dan-I in the first place so that she could take the photo.
Hell, look at how the other girls are insisting that this girl is a real friend to Yeoryeong. It seems like a way of ostracizing Dan-I while making herself a friend to Yeoryeong so that she can also get close to the Four Heavenly Kings.